This invention relates to a method and apparatus for providing an ultrastable and readily adjustable mounting device. More particularly it relates to a mounting apparatus having a particular applicability to mounting two separate and distinct laser emitting devices, electron beam devices, or other extreme precision devices in ultrastable yet readily adjustable relation to one another.
A primary objective of the imaging industry in general is to create images containing a minimum of macroscopic image artifacts or defects. Fidelity of the image is critical to the quality of the graphic art job. Minor deviations in the image can be magnified greatly through the frequency in which they occur. Although these deviations can be in the neighborhood of 1-2 microns, the repetition of a number of these deviations manifests itself as banding of the image or some other visible defect in the finished product. Additionally, the goal of photo microlithography is to create images of the finest detail possible. High resolution images having extremely narrow line and space widths permit the manufacture of both integrated circuitry and electronic circuit boards. As such photoimaging is extensively used in areas where alignment precision within an accuracy of 0.2-0.3 micron is not unheard of.
Although the degree of precision may vary pursuant to the specific industry, the imaging industry in general utilize systems which typically include: an exposure source such as a high intensity lamp, electron beam, laser, or source of other radiation; mask and substrate positioning systems; a projection system to illuminate and image the pattern present on the mask onto a substrate; and an alignment system to accurately position the mask with the substrate prior to exposure. The illumination process is used to either: (1) illuminate portions of the substrate thus exposing these portions; or (2) ablate a coating from regions of the substrate. After one of these or a similar process, the substrate may later undergo further processing or treatment to render the finished product.
Lasers, possessing the requisite degree of precision, are ideal for graphic arts as well as photo microlithography. Over the years the lasers used in such devices have progressed from single beam lasers to multi-beam lasers. Continuous advances in laser technology have enabled ever greater numbers of laser light beams to be coupled into each individual laser head.
Typically, in a multi-beam laser head, each laser beam is located a fixed distance r.sub.h from its neighboring laser beam. To provide adjustability in the placement of each laser beam onto the substrate medium, the entire head is usually capable of rotation. Rotation has the effect of enabling the horizontal distance r.sub.x to vary between r.sub.min and r.sub.max which can range from 0 to r.sub.r. However, any decrease in the horizontal distance r.sub.x necessarily results in a corresponding increase in vertical distance r.sub.y between each laser beam. The effect is governed by the following simple equation: EQU r.sub.x =(r.sub.h.sup.2 +r.sub.y.sup.2).sup.1/2 ( 1)
For example, in some films or plates used in the context of graphic artwork, one possible laser imager design utilizes a cylindrical drum covered with either a photo-sensitive substrate or a substrate with an ablative coating which is rotated at a fixed velocity about its central axis. A laser beam or a number of laser beams are mounted to a carriage assembly which is located at the perimeter of the cylindrical drum and caused to move along the length of the cylindrical drum. In the event a plurality of laser beams are utilized, they are located within a single laser head. Each laser beam is the fixed distance r.sub.h from its neighboring laser beam. By rotating the laser head about its central axis, r.sub.x is reduced from its original dimension of r.sub.h to some desired value. Although vertical displacement of each laser beam is now greater, a computer is used to delay the pulse of each laser beam by a formula which takes into account the velocity of the drum at the drum's surface, the vertical distance r.sub.y between each laser beam, and the distance of each laser from the drum's surface. A carriage upon which the laser head may be mounted is made to move along the length of the drum and adjacent to its surface. The result of sequencing each laser beam pulse while the carriage moves along the drum provides the capability to reproduce any image by a series of microscopic dots. Proper sequencing of each laser beam pulse can provide images with extremely high resolutions and no visible defect or artifacts. In order to maintain the degree of fidelity required in an image, each element in the machine must be precise in its operation. This means that the laser head must be dimensionally stable, the drum must rotate at a velocity precisely correlated to the computer algorithm controlling each individual laser beam pulse, the carriage must move smoothly and the substrate must be accurately positioned on the rotating drum.
Solutions are known in the art to enable such a system to operate adequately. However, at the present time, the maximum number of laser beams which can be used upon devices of this nature are limited by the number of laser beams a manufacturer places into a laser beam head.
To increase production, the Applicants have desired to increase the number of laser beams capable of being used in the generation of a graphic image beyond that currently manufactured in a single head. Since the need to accurately control the placement of each laser beam pulse onto the substrate is critical to the image's fidelity, it was considered impossible to couple two laser heads together without significant deterioration to the fidelity as well as to the accuracy and resolution of the exposure on the substrate. Although each laser beam could be accurately adjusted with respect to an adjacent laser beam contained within the same laser beam head as discussed above, two laser heads could not be accurately adjusted with respect to each other. The problem was that the capability to adjust one laser beam head with respect to the other and to enable them to remain in stable relation simply could not be maintained accurately enough to create an image of the requisite quality. Not only did the mount used to support the laser beam head move under thermally induced expansion and contraction, but drift between the two laser beam heads occurred as well. Any movement between the two translated to a significant alteration in the dimensions r.sub.x and r.sub.y between any two or more laser beams. Though the displacement between each laser beam head appeared minor, the continuous propagation of the error manifests itself macroscopically as banding or some other deterioration in the fidelity ol the image. Furthermore mechanical vibration and drift in placement of the two laser beam heads compound the problem so that it was heretofore unfeasible to couple two laser heads together.